1. Field of the Invention
The invention concerns thermostats actuated by a wax capsule which, by virtue of relative movement between a casing and a piston of the wax capsule caused by thermal expansion of the wax, is adapted in normal operation to open a main valve in a fluid circuit immediately after the temperature of the wax capsule exceeds a predetermined threshold whereas this main valve is closed below this threshold.
2. Description of the Prior Art
The most widespread applications of such thermostats include cooling liquid circuits of internal combustion engines. In this case the thermostat is disposed in the engine cooling liquid circuit so as to bypass a radiator associated with the engine when the temperature of the cooling liquid leaving the engine is below said threshold and otherwise to pass the cooling liquid leaving the engine through the radiator to cool it before it is returned to the engine after cooling.
Thermostats of this kind usually operate satisfactorily but the piston of the wax capsule may jam in its casing in a position such that the main valve is closed even if the temperature of the wax capsule reaches and exceeds said threshold. This can cause serious damage to the engine compromising its service life and/or the manufacturer's warranty, especially in the case of a vehicle engine. If the user of the vehicle whose engine has been damaged in this way makes a claim under the warranty, it is very difficult for the dealer to determine whether the damage is due to failure of the thermostat or to negligence on the part of the vehicle user, who may (for example) have neglected to take note of the cooling liquid temperature indicator lamp and continued to run the engine without taking the action appropriate to such circumstances.
Similar faults can occur in boilers fitted with this type of thermostat, as will be explained later with reference to the figures.
An object of the invention is to provide a safety device for thermostats which operates in a simple and secure way, which alleviates possible failure of the thermostat and in particular of its wax capsule and which provides an indication to the user that failure has occurred.
Another object of the invention is to provide a safety device for internal combustion engines providing an unambiguous indication that damage resulting from overheating of the cooling fluid could have been avoided by the user taking the appropriate supervisory and maintenance action.
With the aim of achieving the above objects the document GB-A-1.091.666 describes a thermostat for an engine cooling circuit which is not actuated by a wax capsule and the main valve of which is provided with an aperture normally closed by a plate held in place by a low melting point alloy. If overheating occurs the alloy melts and opens the aperture by releasing the plate. A safety device of this kind, although it is irreversible as required by the present invention, has the disadvantage that the molten solder may be entrained in the flow and block the engine cooling circuit.
A supplementary object of the invention is to provide the thermostat with an irreversible safety device, that is to say a device requiring action by a specialist to repair or replace the thermostat, without the specialist needing to work on the remainder of the fluid circuit, for example to remove from it solder entrained in the flow as described above.